1. Field of the Invention
The present invention relates to a substrate for an ion conductor, and an ion conductor. The substrate when combined with the salt can be applied to solid conductors for batteries.
2. Description of the Prior Art
There has been expectation that ion conductive polymers can be applied to the field of electrochemistry, for example, to all solid state lithium secondary batteries, etc., because ion conductive polymers are readily processed into thin films, light-weight, and flexible. It is known that polyethylene oxide complexes of alkali metal salts are useful materials to make batteries, because the complexes exhibit ion conduction. Accordingly, systems have been investigated in which alkali metal salts which exhibit good dissociation are dissolved in polymers having polyether chains. Motion of the polyether chains promotes cation conductivity. However, polyethylene oxide exhibits high crystallinity, and low ion conductivity at room temperature. Consequently, in order to improve the mobility of the polymer-chain segments responsible for promoting ion conductivity, comb-shaped polymers have been developed. In the comb-shaped polymers, polymethyl methacrylate, polysiloxane, or polyphosphasene constitutes the main chain, and oligo ether chains are introduced into the main chain to constitute side chains. In particular, in order to reduce the temperature dependence of ionic conductivity, other systems have attracted engineer's attention in which oligoether side chains are introduced into polyether main chains in a dendritic fashion.
In the complexes of ether based polymers and alkali metal salts, however, not only the cations but also the anions are likely to move. When an electrode blocking with respect to anions is used, the anions accumulate at the interface with the electrode and the dc ionic conductivity decreases as time elapses. Therefore, single ion conductors, in which only cations move, are better than other ion conductors in terms of the application of ion conductors to batteries. In order to obtain single cation conduction, it is necessary to fix the anions to the polymer chains. In this case, ion pairing between the cations and fixed anions hinders the motion of the cations, thereby reducing the mobility of the cations. As a result, the ion conductivity decreases sharply in systems where anions, such as carboxylate or sulphonate groups, are introduced into the polymer chains.
It is possible to think of the following counter-measures in order to reduce the influences of the ion pairing: introduction of electron-withdrawing groups into polymer chains to reduce the electron density on the anions; introduction of bulky groups into the polymer chains to sterically hinder the approach of the cations towards the anions; and reduction of the distance between the fixed anions in order to reduce the activation energy required for cation movement.
Several ideas have been proposed to reduce the influence of ion pairing, because the presence of fixed anions reduces the cation mobility as described above. For example, Japanese Unexamined Patent Publication (KOKAI) No. 8-339,827 discloses the introduction of electrophilic groups into polymer chains to attract the electrons at the center of the anions, thereby lowering the electron density at the center of the anions. Accordingly, the electrons are less easily removed from the anions which are thus inhibited from being oxidized. However, the proposal fails to produce a single ion conductor, because the anions are not fixed therein.
In the literature, D. Benrabah, S. Sylla, F. Alloin, J. M. Sanchez, M. Armand, Electrochim. Acta., 40, 2259 (1995) report a system in which lithium sulfonate is fixed to a polymer. The lithium sulfonate is substituted with a fluoroalkyl group, and works as an electron-withdrawing group.
However, these conventional proposals are based on fixing anions onto the polymers. In other words, the resulting polymers have a structure in which the anions are fixed onto the polymers during the synthesis. It is necessary to carry out difficult reactions in order to synthesize the polymers in which the anions fixed therein.